The present invention is in a method for reprocessing zinc-and lead-containing residues from metallurgical plants by a thermal treatment at elevated temperatures and under reducing conditions whereby zinc and lead are volatilized, higher iron oxides are reduced as fully as possible to FeO and zinc and lead vapor are removed from the exhaust gas after it has been cooled.
In the manufacture of pig iron and steel, residues consisting of dusts and sludges are formed in various processing operations, e.g., in the sintering plants, at the blast furnace and in rolling mills. The formed residues consist mainly of iron but contain small amounts of zinc, lead and alkalies and/or are contaminated with oil. The residues cannot readily be recycled to the process operation, e.g., via the sintering plant, because the presence of zinc, lead and alkalies give rise to difficulties in the blast furnace process. Ecological concerns make dumping of such residues increasingly difficult and undesirable. Additionally, considerable amounts of iron, zinc and lead would be lost in a dump. For this reason the contents of zinc, lead, alkalies and oil in such a residue must be decreased before it can be recycled to a sintering plant.
In known processes, zinc, lead and alkalies are volatilized under reducing conditions and a considerable part of the iron oxides is reduced to metallic iron. The sintering of a so-pretreated residue results in the reoxidation of the residue iron metal content. Additionally, the sintering process may be adversely affected by the iron metal contained in the mixture to be sintered.
In other known processes the iron content is reduced only to FeO.
German Patent Publication 10 56 157 discloses a process for treating zinc-containing iron ores in a fluidized bed to form an exhaust gas containing zinc partly as zinc metal and partly as zinc oxide. The iron oxides are at least in part reduced to FeO. However, for a more rapid removal of zinc, the iron oxides must be reduced to iron metal. The zinc-containing iron ores are fed as pellets having a size of the order of millimeters. The fluidized dust is collected from the exhaust gas in a hot cyclone and is recycled to the fluidized bed. The purified exhaust gas is afterburnt with oxidation of zinc metal to ZnO, which is then collected in a dust collector. The reducing gas is introduced from below through a tubular gas feed port. A discontinuous operation has been described because the dezinced material can be discharged through the tubular gas feed port when the gas supply is discontinued.
French Patent Specification 2,373,612 and "Proceedings Ist. Proc. Technol. Conference", Washington (1980) at pages 85 to 103, describe volatization of zinc and lead without the formation of iron metal if coal is not used as a reducing agent. For this reason, dust formed in a metallurgical plant, e.g. of blast furnace dust, and which has a relatively high content of solid carbon is treated to remove the carbon prior to subjecting the dust to a reducing treatment. That carbon removal may be carried out by physical processes. Also the solid carbon content may be almost entirely combusted under oxidizing conditions in a preceding separate stage. The material is treated on a traveling grate or in a shaft furnace. A separate process stage is required for the removal of solid carbon and part of the heat generated by the combustion of solid carbon is lost for the process.
It is an object of the invention to provide a continuous method for the reprocessing of residues from metallurgical plants which permits a reducing treatment in the presence of solid carbonaceous material in conjunction with an effective volatilization.